Fasteners are used in the aerospace industry to mechanically unite various structural components of an aircraft. For example, composite or metal panels that form a portion of a skin of an aircraft wing may be joined to one another via a fastener. In aircraft structures, it is often desirable to install fasteners in interference, meaning that the fastener diameter is larger than the diameter of the hole that receives it. Interference-fit installation of fasteners can facilitate aircraft assembly operations and improve joint performance. As fasteners are intended to enhance the structural strength of an aircraft, it remains desirable to ensure that the act of installing a fastener does not damage underlying structural components of the aircraft. In particular, if an interference-fit fastener is forced through a composite part with too much force, it may cause the composite part to delaminate or experience other issues. Thus, an interference fit fastener may utilize a lubricant that reduces the amount of force used to drive the fastener during installation. Excessive force may also result in damage to the fastener and, in joints comprised of metal and composite parts, it can additionally result in detrimental galling, scoring or excessive deformation of the metal parts. The magnitude of the fastener insertion force may be controlled by the application of lubricants to the fastener or to the hole, by limiting the amount of interference, or by using means other than driving or pulling the fastener into the hole to create the desired state of interference.
Although structural strength of a fastener is important to consider, it is also important for fasteners to adequately conduct and/or disperse electrical energy from the surrounding structure. Hence, it remains important for the fastener to efficiently disperse electrical energy to surrounding structural components in a manner that prevents energy from building at the fastener.
While both structural strength and electrical compatibility remain desirable, it is a complicated process to balance both of these requirements when designing a fastener. For example, a fastener may be coated in order to enhance lubricity and therefore reduce the amount of force used to install the fastener. However, coatings, and/or finishes may electrically insulate the fastener from its surroundings, thereby confounding the ability of the fastener to adequately dissipate electrical energy. As another example, using fasteners sheathed in a protective metal sleeve may provide an adequate level of energy dissipation and reduce the possibility of damaging composite parts during fastener installation. However, such fasteners can be generally costly.
For at least these reasons, designers continue to seek out fastener designs that strike a balance in fulfilling both structural and electrical design constraints.